Shannen Doherty has been publicly battling breast cancer for a year and a half, and not only are her fans in awe of her bravery and openness, so are her good friends and co-workers.

ET's Deidre Behar spoke with Kevin Smith and his daughter, Harley Quinn Smith, on Tuesday at the Burbank studios, and the topic of his previous movies came up while discussing his next film,Yoga Hosers, out Sept. 2. Smith directed Doherty in 1995's Mallrats, crediting the cult classic's existence to the 45-year-old actress.

"I get really emotional about Shannen, because I've been in this business 22 years -- Shannen predates me. She was in this business before I was," he explains. "Without Shannen, I don't get to make Mallrats... The only reason I got to make that goofy a** movie, which was pitched literally as 'Clerks in a mall,' was because Shannen Doherty, who was coming off of 90210, super heat on her, said yes to doing the flick."

When ET spoke with Doherty earlier this month, she mentioned the note from Smith, saying it was one of the funniest messages she received.

"It's such a Kevin Smith thing," she shared. "He sent me this long email, but the beginning of it was, 'You look so effing metal with your haircut.' And it just made me feel so good. I was like, 'Yeah! I'm a bada**!'"

Smith, 46, echoed that same line but also noted her incredible response to a separate part of his email, where he mentioned the project they've been working on: Mallrats The Series. The Tusk director recalls telling his pal, "Get better soon because we're heading towards Mallrats."

Smith has considered working Doherty's illness into the TV show's storyline, but would only do so at her behest. "My suspicion would be [her saying], 'No, dude, I'm dealing with that in the real world. In Mallrats world, let me not have to play with that stuff,'" he says.

Needless to say, Smith is impressed by how Doherty is handling her diagnosis.

"I'm in awe of anybody's strength when they have to face the unknown," he tells ET. "Nobody likes to talk about mortality. We all want to believe we live forever."

"When the real interrupts fake life -- and we all out here [in L.A.] make pretend for a living -- absolutely, it stops everybody," he adds. "And instead of appreciating people like, 'I love that person, I love their movie,' you appreciate the genius of somebody who's facing the unfaceable with absolute grace and good humor, and you're like, 'You're my f**king hero.'"

Coincidentally, Doherty shared a heartbreaking photo to Instagram on Tuesday, showing fans what a day at an oncology center looks like. "The many faces of cancer. Chemo Day," she wrote. "What we call the Red Devil being injected into me via the port thats in my chest. Dawn is my nurse who takes excellent care of me August 10, 2016#cancersucks #fightlikeagirl."